This invention relates to the underwater grading of live farm-grown fish. It has both method and apparatus aspects.
Live fish are graded in order to separate them by size into two groups, or by repetitive grading, into several groups by size.
Several systems have been used for grading live fish that are raised in enclosures, but all these systems have had serious disadvantages or were narrowly limited in their use.
For example, one widely used fish grader requires that a group of fish be taken out of the water, placed into a box having grading openings in its bottom wall, and the box shaken. Fish smaller than a certain size fall out into a pool; then larger are dumped from the box into a different pool. The procedure is hard on the fish; it increases their susceptibility to disease, and may even result in damage to them which reduces their marketability. At the very best, it gives the fish a shock which results in their temporarily losing their appetite and in abnormal action by them after grading.
In another box-type grader, the bottom of the box comprises a series of spaced-apart grading bars. This grader may either be lifted out of the water after the fish have entered it, or it may be left in the water to allow small fish to swim out through the bars. In either case it is inefficient, since it requires that fish be forced into the box in some manner.
Rigid bar graders have sometimes been custom designed as a rectangular grate to be permanently or removably installed in concrete raceways for underwater grading. A seal, to prevent the fish from swimming around the grader, is achieved by designing a channel in the raceway walls into which the ends of the grader fit snugly. Such graders require the close cooperation of the concrete raceways, and they are not, therefore, feasible to install in fish pens of the type where the pen is formed largely by close-mesh flexible netting that is held within a rectangular floating enclosure in a bay or other body of water.
Adjustable rectangular bar graders have also been made wherein the frame and bars were fixed by single pins that allowed the grader to be distorted into a parallelogram. As the grader was distorted into an increasingly acute parallelogram, the spaces between the bars were reduced to provide adjustability. However, it was difficult to prevent the fish from swimming around such a grader.
In some graders, a net was permanently sewed to the perimeter of the grader. Such structures might have the configuration of a net box grader or a seine net with a bar grader sewn in. However, this meant that the grader had to be sewn to the flexible net each time it was used and then disconnected from it after use, or that the grader could be used only with one particular net. Neither of these alternatives is practical for commercial fish pens.
Basically, the present invention is directed to a grading system in which live fish in fish pens can be graded underwater without seriously stressing them or upsetting their feeding, while making it impossible for the fish to swim around or jump over the grading barrier.
An object of the invention is to provide improved underwater fish grading that is easier on the fish than previous systems.
Another object of the invention is to provide underwater fish grading that is readily adaptable to the types of nets that are used in flexible fish pens.
Another object of the invention is to make it easy to adjust to a variety of the sizes of grading.
A further object of the invention is to provide a grading apparatus which does not need to be permanently set in place, or set in place at only one location, but which can be moved from place to place.